Many such devices have been known and are used to automate the sewing together of patterned fabrics, so that the sewing machine operator is relieved of the laborious task of bringing about the desired matching of the pattern structures of the two fabric parts at the sewing site by joining manually and monitoring continuously.
For sewing together according to the pattern, the two fabric parts are usually placed one on top of another such that their faces lie facing one another, and the pattern structures match at least at the site of the seam to be prepared. To accurately bring about this matching automatically, a separate feed system is provided for each fabric part, so that a relative movement between the two fabric parts can be brought about by specifically acting on at least one of these systems in order to compensate for a possible misalignment of the pattern structures. The existence of a misalignment is determined by comparative analysis of two pattern signals which originate from two pattern sensors, each of which scans one of the two fabric parts in order to produce a pattern signal representing the pattern-related variations in light intensity. If only a pattern misalignment in the direction along the seam is determined and corrected, one speaks of one-dimensional misalignment correction. The pattern sensors used for this purpose are usually designed to be such that their field of view has a small extension in the direction of movement of the fabric parts, i.e., in the direction of the seam, and a large extension in the direction perpendicular to that direction, as is described in, e.g., DE 33,46,163 C1. As a result, high resolution is achieved in the direction of movement (scanning direction), while stochastic and systematic effects (e.g., effects caused by longitudinal stripes) of the pattern or fabric structure are reduced at the same time due to summation of the light intensity at right angles to the direction of feed.
Various analytical methods or algorithms are known for determining the degree of matching or the misalignment of the patterns of the two fabric parts from the two pattern signals. In the method described in the above-mentioned DE 33,46,163 C1, the cross-correlation function of the two pattern signals is used to determine the misalignment, and their shift is determined from their normal position (null position). According to another method, known from DE 37,04,824 A1, the pattern signals are differentiated such that sharp needle impulses of one polarity or another are obtained from the ascending and descending flanks of the pattern signals, which correspond to the edges of the pattern components. The two impulse trains thus obtained appear with a phase shift relative to one another in the case of a pattern misalignment, which is determined by superimposing the two impulse trains and measuring the distances between impulses, taking into account the actual polarity of the impulse. It is also possible to superimpose the two pattern signals to be differentiated on one another and to measure the differential surface of the two curves in order to thus determine the pattern misalignment, as is known from, e.g., DE 39,02,473 A1.
Using the above-mentioned one-dimensional misalignment correction, it is possible to align only patterns which are directed at right angles to the direction of sewing, i.e., horizontal or diagonal stripe patterns. However, there are also known devices for sewing together, according to a the pattern, fabric parts which have two-dimensional, intersecting patterns (such as check designs) or patterns which are located in the direction of sewing (such as vertical stripes). As is described in, e.g., DE 37,38,893 A1, two-dimensional image sensors are used for this purpose, each of which is provided for detecting the pattern in a preset rectangular frame. Using a two-dimensional cross-correlation function, the values of the pattern misalignment are then determined in two mutually perpendicular directions from the image data of the two sensors. To correct the pattern misalignment on the basis of the values determined, the two feed systems are designed to be such that they are able to displace the corresponding fabric parts relative to one another not only in the direction of sewing, but also in a direction perpendicular to it (i.e., at right angles to the seam).
It is desirable to design a device for automatic sewing together such that the greatest possible number of types of fabric patterns is able to be processed with it. For this reason, the pattern sensors or the correction device in some prior-art devices can be adjusted to the pattern in question by special settings. For example, it has been known from the above-mentioned DE 33,46,163 C1 that to better detect obliquely extending pattern components, the angular orientation of the linear pattern sensors is adjusted according to the angular orientation of the pattern stripe. The repetition period (register length or distance between stripes) of the patterns to be detected, which differs from one case to the next, is also a specific magnitude to which some known devices can be adjusted prior to the sewing process in order to adjust to it the algorithm used to compute the pattern misalignment (cf., e.g., the above-mentioned DE 39,02,473 A1) or to distribute the feed correction to eliminate the pattern misalignment, in the case of greater register lengths, more uniformly over the sewing section to keep curling of the seam at a minimum, as is known from DE 39,02,474 A1.
There are also patterns whose structures cannot be recognized from lightness values, e.g., when the pattern consists of stripes of two different colors of equal lightness. Such problems can be solved by setting the color selectivity of the pattern sensors, e.g., by inserting a color filter for one of the two colors in the path of rays of these sensors, as is mentioned in the introduction of the above-mentioned DE 39,02,473 A1. According to an alternative described in the same document, color-discriminating sensors, which deliver three pattern signals for three primary color components, are used as the pattern sensors. Of the three component signals, the one which has the highest peak-to-peak amplitude after differentiation, is then automatically used to compute the misalignment.
It is also advantageous if the correction device is able to select between different algorithms to compute the pattern misalignment. Complicated patterns require--for detection and misalignment computation--a relatively complicated algorithm which requires a large computation capacity or a long computation time, while a simpler algorithm, which can be carried out more rapidly, may be sufficient for simple patterns. To take such circumstances into account, a possibility of adjustment for selecting the mode of computation to be applied is provided in the device known from DE 39,02,473 A1.
To carry out the above-mentioned settings in terms of the register length, the angular orientation of the pattern, and the preferable mode of computation, the sewing machine operator must have a sure eye and much experience in order to be able to decide what adjustments to make on the basis of an existing pattern. Skilled manpower is also required, and this is also the time that the sewing machine operator needs between the individual sewing processes to observe and evaluate the pattern and to perform the corresponding adjustments, which is taken at the expense of the sewing time proper.